1. Field of the Invention
The proposed invention relates to a method for forming bridge free silicide, and more particularly to a method not only improves quality of salicide but also provides a self-aligned contact process in the self-aligned salicide process.
2. Description of the Prior Art
Silicide is an important field in contemporary semiconductor fabrication. Advantages of silicide comprise lower contact resistance, withstand high temperature and allowable of self-alignment process. Further, silicides usually are formed by reacting refractory or near noble metals with silicon. Among them are titanium silicide (TiSi2), cobalt silicide (CoSi2), tungsten silicide (Wsi2), platinum silicide (Ptsi2), molybdenum silicide (MoSi2), palladium silicide (PdSi2), and tantalum silicide (TaSi2).
However, owing to the fact that a higher temperature annealing is need to carry out those refractory metal silicide, an unavoidable issue is the bridge phenomena that means the silicide on gate is connected to the silicide on source/drain and then an unexpected short is happened. Sequentially, the bridge phenomena can be further illustrated in following paragraphs.
In conventional salicide process, metal is formed on the gate, the sidewall spacers and the source/drain regions. And then one, two or more annealing processes are performed to react the metal with the polysilicon (silicon) of the gate and the silicon (polysilicon) of the source/drain to form silicide. Following these annealing process, an etching process is performed to remove any unreacted metal.
One of principal functions of sidewall spacers is to separate silicide on the gate from silicide on the source/drain. However, despite the incorporation of spacers, silicide also may form laterally and easily bridge the separation between the gate and the source/drain. Then the gate is shorted to the source/drain, and so-called "bridge phenomena" occurs. In addition, silicon (polysilicon) diffuses into the metal that covers the sidewall spacers and subsequently reacts with the metal.
Moreover, some conditions tend to favor lateral silicide formation. For examples, conventional furnace annealing in an inert gas atmosphere may foster rapid lateral silicide formation. On the other hand, processing in the sub-0.25 .mu.m processing, the minimum gate width may approach or even reach the dimensions of the grain boundaries between the individual grains of the gate, and then these grain boundaries act as natural barriers to silicon diffusion. Another possible reason of lateral silicide formation is that diffusion of silicon (polysilicon) is unavoidable and then unexpected silicide may be formed on sidewall spacers. Obviously, when scale is reduced danger of the unexpected silicide is increased.
In summary, it is beyond any doubt that bride phenomena is serious issue in application of silicide, and then a method to overcome the issue is instantly required.